The recent introduction of advanced sub-micron sized semiconductor devices require reduced critical dimensions and increased packing densities. At these sub-micron sizes and high densities, even defects and imperfections as small as 1 micron and below are problematic and need to be detected and evaluated. Imperfections in the reticle generated by a photographic mask manufacturing process are one source of defects. Errors generated by such a photomask manufacturing process have become an important issue in the manufacture of semiconductor devices at these sub-micron sizes. Defect inspection techniques for masks are therefore becoming to play a more important role in mask making and quality assurance.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask defects, line widths, heights of edge defects and other features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired, and prevents the needless and costly hold up of masks that do meet specification. However, one of the problems of assessing reticle quality at these submicron levels on an automatic inspection system is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment.
Although mask makers typically repair most defects found at early inspection stages, invariably, defects are found at later inspection stages (such as after pelliclization of the mask has occurred). These late stage defects are sized and classified relative to a defect size specification, the size at which device performance is deemed to be affected.
Currently, defects found by automatic inspection tools are classified in one of the following categories by a human operator: (1) a real defect is a hard or soft defect that exceeds the defect size specification, (2) a sub-specification defect is a random or process-related defect below specification that is within a safety margin, and (3) a false defect is a defect detected by the inspection tool with no apparent cause.
Classification of the above types of defects is largely a subjective activity based upon operator skill. However, as defect size specifications diminish, the distinction between real and sub-specification defect classification has become increasingly difficult. For example, as the line width on sub-micron masks approaches 0.1 micron, the ability to measure defect sizes at 1 micron and below becomes very important. Current production machines have an accuracy of 0.1 micron to 0.2 micron, but this is not sufficient.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value for measurement-based defect classification. Consequently, many mask makers have incorporated measurement aids at the inspection station or have moved the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to "eyeball" the boundaries of the defect. This activity is very subjective and can lead to an error in the measurement of the defect.
For example, particle size is conventionally measured by measuring the distance between opposite edges of the particle. Once a defect is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the defect and another cursor on the other side of the defect. The operator must judge for himself the exact boundaries of the defect and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the diameter of the defect. This technique has many disadvantages.
Firstly, this measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the defect. The operator may misjudge the type of a defect, its boundaries, or may simply misplace a cursor even if the defect is visible. The software then blindly calculates the distance between the cursors, without regard for the type of defect, its true boundaries, etc. The above technique may be performed with a standard video microscope and has an accuracy of about 0.1 micron, but is completely subject to the operator's skill level and interpretation. This technique is also unable to calculate an area for a defect.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these smaller and smaller feature sizes. Current techniques do not adequately address the non-linearities associated with such measurements.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the defect, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure defect sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
One approach that has been taken that uses calibration of an automatic inspection system in order to size defects is described in "Characterization Of Defect Sizing On An Automatic Inspection Station", D. Stocker, B. Martin and J. Browne, Photomask Technology and Management (1993). One disadvantage with the approach taken in this paper is that it only provides a technique for measurement of defects of 1.5 microns and greater. Such sizes of defects would produce a linear relationship between reference sizes and actual measured sizes, and the paper does not account for defects less than 1 micron that would produce a non-linear relationship. Also, the technique does not allow for individual calibration data for particular types of defects.
Therefore, an objective feature measurement tool is desirable for use with a photomask inspection tool that can provide reliable and repeatable measurements of defects and other features of less than about one to two times the microscope resolution (or about less than one micron for optical microscopes). It would be especially desirably for such a tool to operate in a fast and highly practical manner in a production environment.
Furthermore, at subresolution sizes it can be extremely desirable to determine the opacity of a feature or defect. For example, measurement of the area or other dimension of a feature will be affected if the feature is not 100% opaque. That is, features that are less than opaque make flux-based measurements of area, diameter, height and/or other dimensions more difficult. For example, a light measurement of a half-transparent spot defect produces more flux from the defect than a perfectly opaque defect; thus, a measured area would appear to be about one-half of the true area. In other words, a less than 100% opaque feature would appear smaller than it truly is. It would be desirable to determine the opacity of the feature or defect to not only correct for a measured dimension of the feature, but also to help determine what the feature is composed of. It would be especially useful to determine the opacity of features having a size that are less than about twice the wavelength being used.
As previously explained, it is desirable to be able to measure the area, and hence the diameter of a feature or defect that is extremely small. Measurement of diameter in this fashion is especially useful when the feature is round, or when it can be assumed that the feature is round. However, features and defects are not necessarily always round; for example, features of about less than 1 micron in size may appear round when in fact they are not due to the blurring effect. In a variety of situations, it can be important to know more specific dimensions of a feature if the feature is not round. For example, given two aluminum lines on a silicon substrate, an edge defect in one line might produce a short across to the other line. However, measurement of the area of this edge defect (to produce an estimated diameter) will not necessarily indicate that the edge defect is in danger of shorting out the two lines. This is because a rather elongated edge defect that is in no danger of shorting the two lines will have the same area as a narrower, taller defect that may in fact produce a short. Determination of the diameter of a defect by measurement of the area of the defect may not always indicate the true height or width of a defect, especially if the defect is not round.
Traditionally, a deconvolution technique using a Fourier transform to a frequency space has been used to measure height and width of features that are less than 1 micron in size. However, this technique has typically not produced accurate results for features of this size. Therefore, in order to more accurately classify features and defects it would be useful to be able calculate the height and width of a feature or defect with a high accuracy. Determination of height and width of features having sizes that are less than about twice the wavelength being used would be especially useful.
It would also be desirable to be able to determine the radius of curvature of a corner of a line, especially at sizes that approach, or are less than, the wavelength of light or the particle beam being used where blurring is a problem.